Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for correcting seismic data, collected with one or more marine seismic sensors, and generating an improved image of the surveyed subsurface.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of the geological structure (subsurface) under the seafloor. The image represents a geophysical parameter of the earth. While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 110 tows plural detectors 112. The plural detectors 112 are disposed along a cable 114. Cable 114 together with its corresponding detectors 112 are sometimes referred to, by those skilled in the art, as a streamer or antenna 116. Vessel 110 may tow plural streamers 116 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to the surface 118 of the ocean. Also, the plural streamers 116 may form a constant angle (i.e., the streamers may be slanted) with respect to the surface of the ocean, or the streamers may have a curved shape, as disclosed, for example, in U.S. Pat. No. 8,593,904.
Still with reference to FIG. 1, vessel 110 also tows a seismic source 120 configured to generate an acoustic wave 122a. Acoustic wave 122a propagates downward and penetrates the seafloor 124, eventually being reflected by a reflecting structure 126 (reflector R). The reflected acoustic wave 122b propagates upward and is detected by detector 112. For simplicity, FIG. 1 shows only two paths 122a corresponding to the acoustic wave. However, the acoustic wave emitted by the source 120 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 120. Parts of the reflected acoustic wave 122b (primary or up-going wave) are recorded by the various detectors 112 (the recorded signals are called traces) while parts of the reflected wave 122c pass detectors 112 and arrive at the water surface 118. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), the reflected wave 122c is reflected back toward the detector 112 as shown by wave 122d in FIG. 1. Wave 122d is normally referred to as a ghost wave (or down-going wave) because this wave is due to a spurious reflection. The ghosts are also recorded by the detector 112, but with a reverse polarity and a time lag relative to the primary wave 122b. The degenerative effect that the ghost arrival has on seismic bandwidth and resolution is known. In essence, interference between primary and ghost arrivals causes notches, or gaps, in the frequency content recorded by the detectors.
Seismic source 120 may be an impulsive source, e.g., an air gun, or a vibratory source, e.g., a vibratory source element as described in U.S. Pat. No. 8,837,259 (herein the '259 patent), the entire content of which is incorporated herein by reference.
The traces recorded by sensors 112 may be used to determine an image of the subsurface (i.e., earth structure below surface 124) and to determine the position and presence of reflectors 126. However, the ghosts disturb the accuracy of the final image of the subsurface and, for at least this reason, various methods exist for removing the ghosts, i.e., deghosting, from the results of a seismic analysis.
Over the years, many algorithms have been developed to suppress the ghost reflections. This process is commonly known in the art as deghosting, ghost wave-field elimination or wave-field separation. Many of these algorithms assume that the reflecting air-water interface 118 is at a constant datum. In fact, various factors (e.g., wind, air pressure, earthquakes, moving vessels, etc.) cause the air-water interface to vary in time and space. The air-water interface is thus better described by a wave than the traditional flat, horizontal, plane. The wave's shape can cause the travel-time (time to travel from the source 120 to sensor 112) of the ghost reflection 122d to deviate from the traditionally calculated travel-time (with flat air-water interface) by an unknown time delay. As a result, the quality of the wave-field separation can be degraded. This affects the quality of further seismic processing and ultimately, the final image of the earth's reflectivity.
A method for determining the air-water interface shape is disclosed in Kragh et al., 2002, “Sea surface shape derivation above the seismic streamer,” 64th Meeting, EAGE, Expanded Abstracts, A007. According to this method, very low frequencies (<0.5 Hz) of the pressure wave-field need to be recorded. These recordings are then used to invert for wave heights. However, recording such low frequencies requires a modified acquisition set up, which is undesirable.
Robertsson et al., 2002, “Rough sea deghosting using a single streamer and a pressure gradient approximation,” Geophysics, 67, 2005-2011, performs a ghost correction on single sensor (pressure) data by estimating the vertical pressure gradient. This process requires continuous measurements of the wave height above the streamer. The approach is limited to frequencies below the first ghost notch.
Amundsen et al., 2005, “Rough sea deghosting of streamer seismic data using pressure gradient approximations,” Geophysics, 70, no. 1, V1-V9, derive an improved estimate of the vertical pressure gradient (from Robertsson et al. (2002)) using a binomial series expansion. As above, this approach requires continuous measurements of wave height along the streamer, which is not easy to obtain.
Orji et al., 2010, “Imaging the sea surface using a dual-sensor towed streamer,” Geophysics, vol. 75, no. 6, pp. V111-V118, image the air-water interface by extrapolating the separated up-going and down-going wave fields (obtained from a multi-sensor streamer) upwards and performing an imaging condition. This imaging step allows for an estimation of wave height.
CGG (which is the assignee of this patent application) has developed over time various techniques for handling a changing air-water interface while acquiring seismic data. Some of these techniques are described in U.S. patent application Ser. No. 13/927,566, Publication No. US 2014/0043936 (reference A herein), U.S. patent application Ser. No. 14/902,619, International Publication No. WO2015/011160 (reference B herein), and International Patent Application Serial Nos. PCT/IB2015/002626 (reference C herein) and PCT/IB2015/001930 (reference D herein).
According to these CGG references, the process of designature (the recorded traces include a combination of the desired earth reflectivity and the source signature, or far-field signature; it is desired to remove the far-field signature from the recorded seismic data, a process known as “designature”) may be applied in 1D (1 dimensional), 2D, or 3D as described by reference (A). The designature approach described in reference (A) may optionally compensate for one or more of the source emissions (e.g., gun response), source array, and free-surface ghost. In the case of acquiring data where the air-water interface may not be adequately described by a horizontal plane, the methodology may be modified accordingly as described in reference (D). References (A) and (D) discuss the case of impulsive sources (e.g., airgun arrays). In the case of non-impulsive emission, e.g., when a vibratory source is used, other designature approaches have been proposed, for example, references (B) and (C).
Reference (D) describes how to compensate for the effects of a static array and static sea datum (which may be valid for an impulsive source). In the case of a non-impulsive source (e.g., marine vibrator), the sea state and/or source array may vary during the emission (which could be from several seconds to minutes or hours in duration).
However, all these methods require continuous measurements of the wave height along the streamer or a substantial modification of the acquisition set up or the use of an impulsive source, neither of which is desired. Thus, there is a need for methods that use the correct for the source depth and/or the source ghost's travel-time, that do not require a modification of the existing acquisition set up and that accounts for the fact that a vibratory source's position changes with the water surface while the seismic waves are generated. Accordingly, it would be desirable to provide systems and methods that have such capabilities.